A typical electromagnetic solenoid has a coil embedded in a solenoid core with a plunger which is acted upon by the magnetic force developed by the coil. Usually, the solenoid core includes a coaxial bore therethrough. The plunger has a rod which is received in the bore in slidable engagement. The plunger usually has a flange upon which the magnetic force interacts. The core and the plunger are constructed from magnetic material such as soft magnetic iron.
When a current is induced within the coil, magnetic flux is developed through a flux path. The flux path is along the core radially outward of the coil, crosses the air gap between the end of the core and the flange of the plunger, traverses the flange radially inward to the plunger rod, which is received within the core, and finally, radially back to the core across another air gap within the bore between the plunger rod and the core.
Where the magnetic flux radially crosses the air gap in the coaxial bore between the plunger rod and the solenoid core, a significant amount of magnetic force is lost. Since the rod and the flange are being moved axially along the axis of the core, the radial flux does not contribute to any force to cause this movement. Therefore, only one half of the total available magnetic flux which is axial in the air gap between the end of the core and the flange, is being used to produce a magnetic force upon the flange. Since magnetic force is proportional to the square of flux within the air gap, the loss of one-half of the total available magnetic flux results in the loss of three-fourths of the available force which may be obtained from such flux.
It would therefore be highly desirable to construct a solenoid which utilizes all the flux crossing any air gap within the solenoid for developing useful force on the plunger.